Presumably, the key step in viral oncogenesis is the interaction between the tumor virus and a cell which causes a breakdown in normal cellular regulation of growth. The long-term objective of this research program is to delineate the nature of this virus-host interaction. An understanding of virus-induced oncogenesis at the molecular level should provide important insights to the moe general problem of human cancers. The immediate, specific aims of this research proposal are as follows. First, the minimal portion of the simian virus 40 genome capable of modifying normal cellular growth will be identified. This study will utilize cloned fragments of viral genome to immortalize primary rat embryo cells. Second, the effect of position in the human genome on the expression of SV40 early genes as well as the effect of inserted SV40 sequences on expression of nonviral genes near the site of integration will be examined. These experiments will utilize linked and unlinked cotransfection of the SV40 early region as well as xanthine guanine phosphoribosyl transferase and thymidine kinase markers. Third, the sequence of amino acids on the SV40 large T antigen responsible for its accumulation in the nucleus will be identified using site-directed deletion mutagenesis. Fourther, the minimal nucleotide sequence required for polyadenylation of SV40 late mRNAs will be investigated by inserting small genomic segments derived from the normal polyA-addition site into an upstream position. Finally, the nature of the primary integration event by adenovirus type 5 into the human cell chromosome will be investigated at the nucleotide sequence level using a plasmid rescue procedure.